[MicrosoftDemangle] fix warn-trailing false positive

[nickdesaulniers]

Summary

A follow up to commit 6bad76c7ae93 ("[Demangle] fix windows tests")
based on @nico's report.

Fixes: #63740

[nickdesaulniers]

dup

llvm/lib/Demangle/MicrosoftDemangle.cpp

 
 char *llvm::microsoftDemangle(std::string_view MangledName, size_t *NMangled,
                               int *Status, MSDemangleFlags Flags) {
   Demangler D;
 
   std::string_view Name{MangledName};
   SymbolNode *AST = D.parse(Name);
   if (!D.Error && NMangled)
-    *NMangled = Name.empty() ? 0 : &*Name.begin() - &*MangledName.begin();
+    *NMangled = Name.empty() ? MangledName.size()
+                             : &*Name.begin() - &*MangledName.begin();
 
   if (Flags & MSDF_DumpBackrefs)
     D.dumpBackReferences();
 
   OutputFlags OF = OF_Default;
   if (Flags & MSDF_NoCallingConvention)
     OF = OutputFlags(OF | OF_NoCallingConvention);
   if (Flags & MSDF_NoAccessSpecifier)

llvm/test/Demangle/warn-trailing.test

 ; RUN: llvm-undname -warn-trailing 2>&1 < %s | FileCheck %s
 
 ?x@@3HAasdf
 ; CHECK: ?x@@3HAasdf
 ; CHECK-NEXT: int x
 ; CHECK-NEXT: warning: trailing characters: asdf
+
+.?AVNet@@
+; CHECK: .?AVNet@@
+; CHECK-NEXT: class Net `RTTI Type Descriptor Name'
+; CHECK-NOT: warning: trailing characters